Optical networks, including passive optical networks, have been proposed as a copper plant replacement for local telephone service. Passive optical networks direct optical signals between a central office or host digital terminal, and network terminal equipment. Passive optical networks require no power or processing in the field to direct such optical signals between network equipment.
Typically, a passive optical network includes a plurality of optical paths extending from the central office to a plurality of remote nodes. Each remote node is further optically connected to a plurality of optical network units, which may suitably be subscribers' terminal equipment. Such networks transmit downstream optical signals from the central office to each remote node, where the signal is then passively split and distributed to the optical network units. In the upstream direction, in other words, toward the central office, the optical network units transmit optical signals to the remote node, where they form a multiplexed signal that is provided to the central office. Lasers are generally used to generate light used to form the transmitted light signals. For a general discussion of such networks, see, for example, Stern et al., "Passive Optical Local Networks For Telephony Applications And Beyond", Electronics Letters, Vol. 23, pp. 1255-57 (1987).
Downstream signals destined for a plurality of optical network units may generally be provided from the central office to the remote nodes using one of two signal arrangements, broadcast signals and switched signals. In a broadcast signal arrangement, a central office broadcasts a downstream optical signal to the remote nodes using, for example, a time-division multiplexing (TDM) protocol. A TDM signal typically includes a frame of information subdivided into time slots, each time slot assigned to an individual optical network unit (ONU). While the entire TDM signal is broadcast to each ONU, each ONU only uses the data from its assigned time slot. In a switched architecture, the central office transmits a separate signal to each ONU. For example, the central office may transmit wavelength division multiplexed (WDM) signals wherein each ONU or subscriber is assigned a wavelength band. In a WDM network, signals destined for each remote node, and ultimately, each ONU, are created by modulating light at distinct wavelengths at the central office. The modulated light signals are then multiplexed onto a fiber and directed to the remote node. A wavelength division demultiplexer (WDM) at the remote node splits and distributes the downstream signals to the ONUs as a function of wavelength. One notable difference between the two architectures is that in broadcast signal systems, the data signal for all ONUs is provided to each ONU, which results in wasted power and lacks signal privacy. In WDM signals, each ONU sees only its own data signal, which is more power efficient and private.
RITE-Net.TM. is a WDM network in which a central office provides multiplexed optical signals encoded with information at specific wavelengths to each remote node. At each remote node, the downstream signals are routed by wavelength to the ONUs. At each ONU, the received light is overmodulated with upstream information and looped back through the remote node to the central office. To this end, a modulator at the optical network unit imprints information on a portion of the downstream signal which is then directed back to the remote node. The RITE-Net.TM. network not only has the privacy and efficiency advantages of WDM optical communication, but further has the advantage of not requiring a light source at the ONU. From a network standpoint, it is advantageous that the ONUs cannot generate their own light signals. RITE-Net.TM. is described in commonly-owned U.S. patent application Ser. No. 08/029,724, filed Mar. 1, 1993, and incorporated herein by reference.
At present, however, the multiwavelength laser and modulator technologies that are preferably employed in a RITE-Net.TM. network are still being developed. In the interim, broadcast TDM networks have been proposed that utilize widely available technology.
One design consideration in passive optical networks is the facility for diagnostics which may be used to identify and locate network component failures, such as fiber cuts or transmission equipment failure. For cost efficiency, passive optical networks, including both broadcast signal and switched signal networks, are preferably capable of performing diagnostic operations from the central office location. One known diagnostic operation that may be performed at the central office is the optical time domain reflectometry ("OTDR") test, which is described, for example, in J. M. Senior "Optical Fiber Communications" pp. 822-27 (Prentice Hall 1992).
In the OTDR test, a light pulse is provided at one end of the fiber, typically at the central office. Then, a measure of the light reflected back due to backscatter effects is taken. The measure of backscattered light provides an indication of the status of the optical link to the node. In practice, if a particular ONU cannot communicate with the central office, the OTDR test may be used to determine if there is a fiber cut, and if so, its approximate location.
The OTDR test in some circumstances does not adequately identify a network failure. Consider, for example, a situation in which a particular ONU is not communicating. In such a situation, the cause may be an optical path problem such as a fiber cut, or a failure in the ONU transmission or reception equipment. If the OTDR test is performed and the results indicate that their may be a fiber break at or near the ONU, there is no way to distinguish between a fiber failure near the ONU or an ONU transmission or reception equipment failure.